A ring-shaped member 1 as shown in FIG. 12 has been conventionally manufactured through a method of punching out a plurality of annular ring-shaped members 1 by one column or a plurality of columns by a pressing device from a raw material 2 having a predetermined plate thickness as shown in FIGS. 13(a) and 13(b).
However, in the conventional manufacturing method, the yield rate or the percentage of the weight of the ring-shaped member 1 with respect to the weight of the raw material 2 is bad, and tends to increase the cost of the ring-shaped member 1.
This is because the material is removed at high percentage of skeleton S1 and slug S2 with respect to the ring-shaped member 1. The “Skeleton” is a frame shaped scrap that remains after punching out a plurality of ring-shaped members 1, and the “slug” is a plurality of scraps punched out and removed with a punch when punching out the plurality of ring-shaped members 1.
A method of molding the ring-shaped member shown in FIGS. 14(a) and 14(b) has thus been proposed in Japanese Published Patent Application No. 62-203633.
The method of molding the ring-shaped member described in Japanese Patent Application No. 62-2036331 includes a step of continuously bending the width of an elliptical ring-shaped raw material into a circular shape while preventing the deformation thereof, and a step of making the raw material formed into a circular shape into a perfect circle.
That is, as shown in FIG. 14(a), inner and outer rollers 103, 104 that freely rotate and move so as to slidably hold in between one width on the minor side of the elliptical ring-shaped raw material 100, and a pair of freely movable bending rollers 105 are arranged, where the inner and outer rollers 103, 104 are rotated in a direction of an arrow F or in the opposite direction thereof to push the pair of bending rollers 105 in the direction of an arrow G while feeding the elliptical ring-shaped raw material 100 in the direction of an arrow L or in the opposite direction thereof, thereby sequentially performing the curvature process on the linear portion of the raw material 100 to be molded into a raw material 101 of circular shape.
Thereafter, as shown in FIG. 14(b), a large diameter inner roller 106 is externally fitted to the inner roller 103 so as to be simultaneously rotatable and inscribed to the raw material 101 formed into a circular shape, and the large diameter inner roller 106 and the outer rollers 103, 104 are rotated in the direction of the arrow F or in the opposite direction thereof to obtain a ring-shaped member 102 molded to a perfect circle.